The present invention generally relates to an optical filter and, more particularly, to a grating-based optical filter for attenuating the amplitude of an optical signal to provide a desired spectral profile.
Optical amplifiers continue to be the catalyst of the rapid growth of all optical networks. With dynamic reconfigurability looming on the near term horizon, the performance demands on amplifiers will become even more stringent. In the simplest of the amplifier designs where the amplifier gain is attenuated by one of a variety of techniques, the gain is not constant throughout the spectrum but has a specific profile determined by the operating conditions. This non-uniform gain spectra leads to non-uniformity in the optical signal powers in dense wave division multiplexing (DWDM) channels, which, left unchecked, can result in an increased signal-to-noise ratio, bit error ratio, or channel loss in the optical amplifier signal.
As known, gain equalization filters are used to selectively attenuate an optical signal to compensate for the gain variations produced by the amplifier, and thereby flatten the amplifier gain spectra. A variety of technologies have been proposed, including those based on arrays of microelectro-mechanical systems (MEMS) cantilevered beams and/or mirrors, Faraday rotation elements, long period fiber gratings, thermally controlled array waveguides (AWGs), LCD technology, and acousto-optic tunable filters. Long period fiber gratings typically couple light from a core mode to a cladding mode and have a grating period of 10-100 times greater than a short period grating (which may typically be 1 micron).
Some disadvantages of these technologies include the fact that long period fiber gratings result in a long device, which are cumbersome to compensate for temperature. Other devices involve sensitive free space optical path and bulk optic elements, all of which are extremely alignment sensitive, thermally unstable and bulky. Furthermore, most of these technologies do not take into consideration the wavelength shifts in the gain spectra due to temperature changes and other factors.
In addition, it has long been thought that slanted or xe2x80x9cblazedxe2x80x9d Bragg gratings would make ideal gain flattening filters because the blazed Bragg gratings can, in principle, be designed to provide wavelength dependant loss, like a thin film filter, but without the back reflections common in conventional Bragg gratings. It has been said that the use of these gratings in a product would be considered disruptive, because one would get the best of both worlds, the performance of Bragg gratings, but with the reflection suppression qualities of thin film filters. Another key advantage of Braggs over thin films is the speed with which new designs can be turned around.
In a paper xe2x80x9cNew and efficient technique for the suppressing the peaks induced by discrete cladding mode coupling in fiber slanted Bragg grating spectrumxe2x80x9d, which is incorporated herein by reference, the author describes a method for overcoming a significant technical constraint of the technology, which is the coupling to discrete cladding modes. Because the author""s experiments were conducted in fiber, where the cladding diameter is 125-micron, there were a finite number of cladding modes, which interfered to form a series of peaks in loss spectrum, which is very undesirable. The paper explains that the best way to circumvent this issue is to surround the fiber with an xe2x80x9cinfinitexe2x80x9d index-matched medium, which results in a continuum of cladding modes and therefore a smooth loss spectrum. Materials that were cited, include, polymers and oil. However, the long-term reliability of such materials could not be verified.
Consequently, two alternative solutions were proposed, chirping the grating or making the grating short. Each of these techniques is designed to cause the cladding modes to smear into each other so as to prevent ripple in the loss spectrum. These solutions, however, come at a cost. In the case of the chirped grating, one may lose the ability to write a sufficiently sharp spectral profile because the grating must necessarily be broadened to prevent the cladding modes. In the case of shortening the grating, one makes two sacrifices. Shorter gratings have broader spectral profiles, and again one may give up the possibility of writing sufficiently sharp gratings. Also, and probably the most significant sacrifice, is that shorter gratings require much larger index changes to achieve the same loss values. The paper illustrates 0.7 mm long gratings, which for many applications would not allow for a sufficiently strong grating with the sort of index modulations that are achievable with conventional writing and hydrogen loading techniques.
Thus, it is advantageous and desirable to provide an optical filter for flattening the gain of an optical signal, which overcomes these disadvantages.
An object of the present invention is to provide an optical filter, including an optical waveguide with a reflective elements disposed therein, for altering the spectral profile of an optical signal to a desired gain profile (e.g. a flattened gain profile), wherein optical waveguide has characteristic that permit the spectral profile of the optical filter to be altered.
In accordance with an embodiment of the present invention, an optical filter comprises an optical waveguide that includes an outer cladding having a core disposed therein. The waveguide has an outer waveguide dimension of the waveguide is greater than 0.3 mm. A slanted grating is imparted in the waveguide for selectively attenuating a received optical input signal to provide an optical output signal having a desire spectral gain profile.
In accordance with an embodiment of the present invention, an optical filter comprises an optical waveguide that includes an outer cladding with a core disposed therein. The waveguide has an outer waveguide dimension of the waveguide is greater than 0.3 mm. A reflective element is imparted in the core of the waveguide for selectively attenuating a received optical input signal to provide an optical output signal having a desire spectral gain profile.
In accordance with an embodiment of the present invention, an optical device comprises an optical waveguide having an outer cladding with a core disposed therein. An outer waveguide dimension of the waveguide is greater than 0.3 mm. A reflective element is disposed in the waveguide to minimize back reflection.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.